Hypothes.is is the leading online open platform for annotating the web, while Readium is the largest open source project for consuming ePUB documents.

The goal of this partnership is to bring ebook annotations capabilities to the web. This is what Laurent Le Meur of EDRLabs had to say in A partnership to bring open annotation to eBooks – EDRLab :

Digital books represent an enormous class of content which at present cannot be collaboratively annotated with others. Combined with the recent work that the W3C has done to standardize annotation, this represents an essential next step in bringing a high quality open annotation implementation to books everywhere.

We’re planning a more technical writeup to highlight the integration work that’s been done in the future. We are currently working on a release with a target in summer of 2017.